The Origin and Evolution of Mammals - Moodle

42 THE ORIGIN AND EVOLUTION OF MAMMALS
Copper sandstone lower jaw as the type and
only safely attributed specimen of V. prima. Thus,
although the family Venyukoviidae still stands,
Venyukovia itself is scarcely known.
Meanwhile, Tchudinov (1960, 1983) had described
another primitive anomodont as Otsheria netzvetajevi,
based on a skull without the lower jaw (Fig. 3.11(f)).
This specimen is older than the Isheevo material, for
it was found at the Ezhovo (Ocher) locality which is
dated as late Kazanian. Most recently a fourth, quite
different form, Suminia (Fig. 3.11(g)), has been
described from the Late Tatarian Kotel’nich fauna,
which is therefore the youngest of the Russian primitive
anomodonts (Ivakhnenko 1994; Rybczynski
2000).
A cladistic analysis by Modesto and Rybczynski
(2000; Rybczynski 2000) concludes that the Russian
genera are a monophyletic group characterised
principally by an elongated postero-dorsal process
of the premaxilla, a very broad parietal bone with
the pineal foramen situated on a large boss at the
front of it, and no preparietal bone.
Otsheria (Fig. 3.11(f)) is thus the earliest of the at
least reasonably well-known genera of venyukovioids
(Tchudinov 1960, 1983). It has a small, 10-cm long
skull that differs mainly from other venyukovioids in
its dentition. Although not very well preserved, this
can be seen to consist of probably four incisors in the
premaxilla and nine maxillary teeth, all of which are
short and stout, with laterally compressed points. In
the absence of any information at all about the lower
dentition, it is not clear whether there was tooth-totooth
contact. So far as it may be inferred, the form of
the individual upper teeth suggests a simple, foliagecutting
action, although a generalised omnivorous
diet might well have been the case.
Ulemica (Fig. 3.11(e)) is based on two fairly complete
skulls, several partial skulls and lower jaws, but
so far no postcranial material, and all from the Early
Tatarian Isheevo locality (Ivakhnenko 1996). The
skull is 15–20 cm in length and heavily built. The dentition
of Ulemica is very odd and difficult to understand
functionally (King 1994; Ivakhnenko 1996). In
the upper jaw, there is a large chisel-shaped first
tooth, followed by three of similar size but with heellike
internal extensions. These are followed by four
small conical teeth and a much larger, swollen,
caniniform tooth. Finally the tooth row is completed
by a series of four or five very small pointed teeth.
The corresponding lower dentition consists of a
chisel-shaped first tooth, followed by three smaller
teeth that are laterally flattened in younger specimens,
but replaced by blunter, rounded ones in older
ones, and apparently lost in fully grown specimens.
The tooth row is completed by a series of small,
medially displaced teeth. The biting action consists of
direct tooth-to-tooth contact between the anterior
teeth. The upper caniniform tooth does not meet a
lower tooth, but bites into a pit on the bony surface of
the jaw, to the side of the lower teeth. Behind this
point, the teeth do not appear to meet either. The
upper postcaniniform teeth bite towards the lateral
side of the lower teeth. The latter, in turn, bite
towards the bone of the palate, medial to the upper
teeth. King (1994) suggested that horny biting surfaces
were present in life on these bony surfaces.
Judging both from the form of the dentition and the
structure of the articular and quadrate, no propalinal
movement of the lower jaw was possible. Instead, the
anteriormost teeth would have provided a nipping,
food-gathering action, and the posterior teeth acting
against their respective opposing tooth plates would
have provided a simple crushing function. The diet
for which such a system was adapted is not clear, but
presumably included various kinds of vegetation.
Suminia (Fig. 3.11(g)), as well as being the youngest
of the primitive Russian anomodonts is also the
best known, on the basis of several skeletons and
skulls (Ivakhnenko 1994; Rybczynski 2000). The
skull is small, no more than about 6 cm in length,
and unlike Ulemica it is lightly built. There are complete
upper and lower marginal dentitions of 10 or
11 teeth, which decrease in size gradually from
front to back. Each of the teeth has a broad base and
a serrated, leaf-like crown, uppers with a concave
front edge, lowers with a concave hind edge.
Rybczynski and Reisz (2001) have analysed the jaw
action, showing that the power stroke of the lower
jaw was posteriorly directed and created a cutting
action between lower and upper teeth that included
direct tooth-to-tooth contact. Their interpretation
is supported by the structure of the jaw hinge, which
permitted propalinal movement of the jaw, and
by the lateral flaring of the zygomatic arch, the
squamosal component of which bears a lateral fossa
for the origin of a lateral slip of the adductor